1. Field of the Invention
The present invention relates to an exchange-coupled film which includes a seed layer, an antiferromagnetic layer, and a ferromagnetic layer disposed in that order from the bottom and in which the magnetization direction of the ferromagnetic layer is pinned in a predetermined direction by an exchange coupling magnetic field generated at the interface between the antiferromagnetic layer and the ferromagnetic layer; a method for making the exchange-coupled film; and a magnetic sensing element including the exchange-coupled film. More particularly, the invention relates to an exchange-coupled film in which the total thickness and shunt loss can be decreased by decreasing the thickness of the seed layer thereof, a method for making the exchange-coupled film, and a magnetic sensing element including the exchange-coupled film.
2. Description of the Related Art
FIG. 27 is a partial sectional view of a conventional magnetic sensing element (spin-valve thin-film element), viewed from a surface facing a recording medium.
As shown in FIG. 27, an antiferromagnetic layer 50, a pinned magnetic layer 51, a nonmagnetic layer 52, a free magnetic layer 53, and a protective layer 47 are disposed in that order on a seed layer 44 which is, for example, composed of a NiFeCr alloy.
In such a spin-valve thin-film element, an exchange coupling magnetic field is generated at the interface between the antiferromagnetic layer 50 and the pinned magnetic layer 51 by annealing, and the magnetization of the pinned magnetic layer 51 is pinned in the height direction (in the Y direction in the drawing).
In the spin-valve thin-film element shown in FIG. 27, hard bias layers 5a are disposed at both sides of a laminate including the seed layer 44 to the protective layer 47, and the magnetization of the free magnetic layer 53 is aligned in the track width direction (in the X direction in the drawing) by a longitudinal bias magnetic field from the hard bias layers 5a. 
As shown in FIG. 27, electrode layers 8a are disposed on the hard bias layers 5a. A sensing current from one of the electrode layers 8a mainly flows through three layers, i.e., the pinned magnetic layer 51, the nonmagnetic layer 52, and the free magnetic layer 53.
In the spin-valve thin-film element shown in FIG. 27, by providing the seed layer 44 under the antiferromagnetic layer 50, an improvement in current-carrying reliability, for example, electromigration resistance, and an improvement in the rate of change in resistance are expected.
It is believed to be important that the seed layer 44 has a face-centered cubic crystal structure (fcc structure) and the equivalent crystal plane represented as {111} plane is preferentially oriented parallel to the layer surface.
If the seed layer 44 has the fcc structure with the {111} orientation, the individual layers can be formed on the seed layer 44 properly so as to have the {111} orientations of fcc structures. It is also possible to increase the crystal grain sizes. Consequently, scattering of conduction electrons in the grain boundaries can be reduced, resulting in an improvement in electrical conduction. The magnitude of the exchange coupling magnetic field generated between the pinned magnetic layer 51 and the antiferromagnetic layer 50 can also be increased, resulting in an improvement in current-carrying reliability.
Conventionally, the seed layer 44 is composed of a NiFeCr alloy or NiCr alloy as disclosed in Japanese Unexamined Patent Application Publication No. 2003-101102.
Recently, there has been an increased demand for gap-narrowing and decreases in shunt loss in magnetic sensing elements, and further decreases in the thicknesses of the individual layers constituting magnetic sensing elements have been required.
The conventional seed layer has a thickness of about 50 Å. If the thickness of the seed layer is decreased so as to be smaller than the critical thickness, the magnitude of the exchange coupling magnetic field Hex between the antiferromagnetic layer and the ferromagnetic layer disposed on the seed layer is extremely decreased, resulting in a rapid degradation in the change in resistance ΔRs and the rate of change in resistance ΔRs/Rs.
Laminates having the structure descried below were formed for testing. Changes in the rate of change in resistance ΔRs/Rs, change in resistance ΔRs, sheet resistance Rs, and unidirectional exchange bias magnetic field Hex* with the thickness of the seed layer composed of (Ni0.8Fe0.2)60Cr40 were investigated.
Substrate/Alumina/Seed layer [NiFeCr]/Antiferromagnetic layer [PtMn (140 Å)]/Pinned magnetic layer [CoFe (16 Å)/Ru (8.7 Å)/CoFe (22 Å)]/Nonmagnetic layer [Cu (21 Å)]/Free magnetic layer [CoFe (10 Å)/NiFe (35 Å)]/Protective layer [Ta (30 Å)]
The results thereof are shown in FIGS. 28 to 31. FIG. 28 is a graph showing the measurement results of the sheet resistance Rs; FIG. 29 is a graph showing the measurement results of the unidirectional exchange bias magnetic field Hex* of the pinned magnetic layer; FIG. 30 is a graph showing the measurement results of the rate of change in resistance ΔRs/Rs; and FIG. 31 is a graph showing the measurement results of the change in resistance ΔRs.
As shown in FIGS. 28 and 29, if the thickness of the seed layer becomes 38 Å or less, the sheet resistance Rs of the magnetic sensing element rapidly increases, and the unidirectional exchange bias magnetic field Hex* rapidly decreases. As shown in FIGS. 30 and 31, if the thickness of the seed layer becomes 38 Å or less, the rate of change in magnetoresistance ΔRs/Rs and the change in resistance ΔRs rapidly decrease.
The thickness of the seed layer at the critical point below which the unidirectional exchange bias magnetic field Hex*, change in resistance ΔRs, and rate of change in resistance ΔRs/Rs of the laminate constituting the magnetic sensing element rapidly decrease is referred to as a critical thickness of the seed layer.
As described above, because the seed layer has the critical thickness below which the change in resistance ΔRs and the rate of change in resistance ΔRs/Rs rapidly decrease, it has not been possible to form an exchange-coupled film in which the thickness of the seed layer is smaller than or equal to the critical thickness or a magnetic sensing element including such an exchange-coupled film.